Journal of Thermal Analysis and Calorimetry, Vol. 60 (2000) 97–102

THERMAL ANALYSIS OF BINARY SYSTEMSExplosive – lead compound

A. Ksi¹¿czak1, A. Maranda2 and D. Rosenkiewicz2

1Department of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw2Department of Armament and Aviation, Military University of Technology, Kaliskiego 201-489 Warsaw, Poland

Abstract

The thermal decomposition of explosives: pentaerythrol tetranitrate (PETN), 2,4,6-trinitrotoluene(TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitroamine (RDX) and their two-component mixtures with40% of lead compounds [PbO, Pb3O4, Pb(NO3)2] were performed. The simple method of determina-tion of stability changes in the mixtures described above, in comparison with pure explosives waspresented. The lead oxides accelerated significantly the thermal decomposition of explosives.Pb(NO3)2 acts as a catalyst in the mixture containing TNT degradation, but not in a case of PETNand RDX.

Keywords: compatibility, explosives, lead compounds, thermal decomposition

Introduction

The explosive mixtures consisted of a basic component – sensitiser, as well as othermodifiers, changing their physicochemical properties and utility. The lead tetraoxide(Pb3O4) is described as a modifier in some patents [1, 2]. The addition of this com-pound into plastic explosives causes the decrease of detonation velocity. Because ofextremely high temperatures and pressure in detonation wave and high dynamics ofprocess, there are some difficulties in investigating the chemical reactions accompa-nying the process. The information of interaction between the components of a mix-ture can be obtained by comparison of the thermal stability for pure compounds andtheir mixtures.

DSC measurements are the good tool for the determination of stability of mix-tures because the thermal decomposition of explosives is a strongly exothermic pro-cess. There were performed the stability tests for two-component mixtures of explo-sives like pentaerythrol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT), and cyclo-1,3,5-trimethylene-2,4,6-trinitroamine (RDX) containing 40% (mass) of lead com-pounds; PbO, Pb3O4, Pb(NO3)2. The relative thermal decomposition rate was esti-mated by using the initial stage of thermal decomposition for the small degree of con-version. The method of quick estimation, the influence of lead compounds on the sta-bility of compounds tested, based on the initial stage of thermal decomposition waspresented.

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Experimental

The DSC measurements were performed in non-isothermal mode with linear scan-ning rate β=2 K min–1. The calorimeter was calibrated using the pure In, Sn, Pb, Bi,Zn as a calibrant [3, 4]. The samples of mass about 10 mg were closed hermeticallyunder vacuum in alumina pans. The measurements were performed until the firstsigns of unsealing appeared. The experimental points before unsealing were used tocompare the decomposition rate between pure explosive and its mixture. The unseal-ing of the sample pan during the measurement is visible on the thermoanalyticalcurve as a sharp endotherm caused by the relaxation of gaseous products. Our ownsoftware was used [3] for the calculation of thermal effects and kinetics parameters.All substances used in the measurements were not less than 99.0%. The explosivemass of the sample used to DSC measurement was always the same.

Results and discussion

The process of decomposition of explosives is complicated [4]. The degradationproducts could undergo further degradation and possess the catalytic properties. It isconsidered in the simplest model of decomposition, that the process is the first-orderreaction and the products act as catalysts of the decomposition (autocatalytic reac-tion). The total reaction rate could be expressed by reaction:

Vt

k k=d

d= (1– )+ (1–1 2

α α α)α (1)

where α is a degree of conversion, k1, k2 – the rate constants for the first-order andcatalytic reactions, respectively. For the very small degree of conversion α<<1 theEq. (1) could be written as:

d

d= +1 2

α αt

k k (2)

The solubility of volatile products in established temperature is constant. Theconcentration of catalytic volatile products undergo the relation:

α=α*=const. (3)

Substituting Eq. (3) into (2) we obtained:

d

d= + = =const.1 2

α αt

k k k (4)

For the small degrees of conversion we can consider that reaction rate is constant.The kinetics and thermodynamics factors have an influence for this value [5, 6].

The beginning of decomposition of explosive is visible as a deviation of baselinetowards the exothermal effect on the thermoanalytical curve. The rate of decomposi-

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tion is proportional to the deviation of the calorimetric signal from the baseline (b)which is proportional to the released heat (q):

γ σ αb

q

t tV= = =d

d

d

d(5)

where γ and σ are the constants. The equation, describing the relative rate of decom-position ω is:

ω= = =b

b

V

V

k

k0 0 0

(6)

where b is the deviation of the calorimetric signal for the mixture and bo for the pureexplosive. The Eq. (6) is applicable for isothermal and non-isothermal conditions ofmeasurements. When the scanning rate β is linear the relation is given by Eq. (7).

d dT t=β (7)

Figure 1 presents DSC curves of melting and the beginning of decomposition ofPETN and mixture 60% PETN and 40% Pb(NO3)2. It can be seen that this lead com-pound gives no accelerating effect on decomposition process (ω 1≈ ). The comparedcurves are very similar.

The DSC curves of melting and the beginning of decomposition of PETN (A),mixtures PETN/Pb3O4 60/40 (B) and PETN/PbO 60/40 (C) were presented in Fig. 2.It shows that stability changes according with sequence A>C>B. The rate of thermaldecomposition for tested systems was compared for the least temperature, in whichthe unsealing of the pan is observed (T=439 K). As basic values, the extrapolated val-ues of baseline lead from the linear area before melting until the maximum effect ofdecomposition was accepted. The ratio of b/bo at the same temperature (T=439 K) isequal to the relative rate of decomposition. The calculated results of relative rate of

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KSI¥¯CZAK et al.: THERMAL ANALYSIS OF BINARY SYSTEMS 99

Fig. 1 Comparison of the melting and beginning of thermal decomposition DSC curvesfor pure PETN –– and PETN + 40% Pb(NO3)2 mixture - - -

decomposition (ω) is collected in Table 1. It was proved that lead oxides acceleratedthe decomposition and the better catalytic properties possess Pb3O4.

Table 1 Relative rate of decomposition (ω) at 439 K for mixture containing 60% PETN

Mixture Relative rate of decomposition (ω)

PETN+Pb(NO3)2 ~ 1

PETN+PbO 1.33

PETN+Pb3O4 1.57

Table 2 Relative rate of decomposition (ω) at 511 K for mixture containing 60% TNT

Mixture Relative rate of decomposition (ω)

TNT+PbO 2.33

TNT+Pb3O4 2.66

TNT+Pb(NO3)2 2.81

The pure TNT and their mixtures of lead compounds were also investigated. TheDSC curves of melting and the beginning of decomposition of TNT were present inFig. 3. The values of relative rate of decomposition (ω) were calculated for the 511 Kas the highest temperature of unsealing the pan. Results were presented in Table 2. Itis worth mentioning that all investigated lead compounds accelerate the decomposi-tion in the case of TNT. The differences of relative rates of decomposition (ω) for in-vestigated TNT mixtures are low. It is probable that significant growth of ωvalues forTNT mixtures in comparison with PETN mixtures is caused by the higher value ofend temperature of the process. The DSC curves for TNT decomposition the traces of

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Fig. 2 Comparison of the melting and beginning of thermal decomposition DSC curvesfor pure PETN ––, mixture with 40% PbO - - - and mixture with 40% Pb3O4 · · · ·

degradation were not observed at T=439 K. The rate constant in this temperature isseveral times lower than in the case of PETN.

The DSC curves of RDX and their mixtures with lead compounds are shown inFig. 4. The shape of DSC curves of the decomposition of pure RDX and mixtures withPb(NO3)2 is similar and it proves that the presence of lead nitrate has no influence onthe decomposition of explosive. In both cases one can observe the melting peak andstrong decomposition exotherm. The estimated enthalpy of melting of RDX was∆Hm=20.5 kJ mol–1, melting temperature Tm=476.5 K. The literature parameters [7]were ∆Hm=35.76 kJ mol–1, Tm = 477.2 K respectively. The low values of the melting

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Fig. 3 Comparison of the melting and beginning of thermal decomposition DSC curvesfor pure TNT ––, mixture with 40% Pb(NO3)2 - - -, mixture with 40% Pb3O4 · · · ·and mixture with 40% PbO – · · –

Fig. 4 Comparison of the melting and beginning of thermal decomposition DSC curvesfor pure RDX ––, mixture with 40% Pb(NO3)2 - - -, mixture with 40% Pb3O4 · · · ·and mixture with 40% PbO – · · –

enthalpy is the result of the great exothermic effect in the melting process. The resultsof the comparison of DSC curves of melting and decomposition of pure hexogene andits mixture with PbO (60/40) show that PbO acts as a catalyst for RDX decompositionin the solid phase. The exothermic effects appear before melting and they were over-lapping. The enthalpy of this process is ∆HD= –3.8 kJ mol–1 and the melting enthalpyis ∆Hm=7.6 kJ mol–1. The very low values of melting enthalpy shows that the effect ofdecomposition, induced by catalytic properties of PbO is great. The comparison of thebaselines before the decomposition lead us to the conclusions that this effect appearsat much lower temperatures. The mixture RDX/Pb3O4 60/40 gives a low exothermiceffect ∆HD= –2.0 kJ mol–1 and the value of melting enthalpy is in agreement with ex-pectations and is ∆Hm=8.6 kJ mol–1. The calculations of relative reaction rates forRDX and mixtures with lead oxides were performed except for mixtures withPb(NO3)2. In this case the decomposition does not occur before melting. The relativevalue of ωfor the mixture RDX/PbO, compared with the value for the mixtureRDX/Pb3O4 is 1.71. The shifting of the melting peak toward the direction of lowertemperatures is caused by the presence of dissolved thermal decomposition productsof RDX. The dislocation of melting peaks is in agreement with exothermal effects inthe solid phase. DSC curve for RDX is very similar to its mixture with Pb(NO3)2. Thisresults indicate that Pb(NO3)2 is an inert substance for RDX.

Conclusions

This method of estimation, the relative rate of thermal decomposition of pure explo-sives allows the quick determination of the thermal compatibility of components. Theresults lead us to the conclusion that lead oxides have catalytic activity in the processof decomposition of all investigated explosives. Their activity is strongly determinedby physicochemical properties of the given explosives. The observations suggest thatlead compounds have influence on chemical reactions during detonations. The leadnitrate does not catalyse the decomposition reaction of PETN and RDX, but acts as acatalyst in the decomposition of TNT.

References

1 R. S. Gow, Explosive compositions, patent GB 089403, 1967.2 W. E. Schulz, Process of producing a fibrous explosive having variable density, US patent

3102833, 1963.3 A. Ksi¹¿czak and T. Ksi¹¿czak, J. Thermal Anal., 43 (1995) 79.4 A. Ksi¹¿czak and T. Ksi¹¿czak, Thermochim. Acta, 268 (1995) 95.5 A. Ksi¹¿czak and T. Ksi¹¿czak, Thermochim. Acta, 275 (1996) 27.6 A. Ksi¹¿czak and T. Ksi¹¿czak, Thermochim. Acta, 284 (1996) 299.7 J. Kohler and R. Meyer, Explosives, VCH Verlagsgesselschaft, Weinhem, 1993.

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